The invention relates to the field of digital subtraction angiography. The invention disclosed herein pertains to methods and apparatus for performing temporal and hybrid subtraction of x-ray images which results in improved signal-to-noise ratio.
The new hybrid subtraction method and apparatus are used in an x-ray procedure wherein the interior configuration of a blood vessel is delineated by having an x-ray contrast medium flowing through the vessel in the course of obtaining a sequence of x-ray images of the vessel.
The basic hybrid subtraction method and apparatus for performing the same are described in pending patent application Ser. No. 260,694, filed May 5, 1981 wherein W. R. Brody is the inventor. Hybrid subtraction involves a combination of temporal subtraction and energy subtraction methods.
Temporal subtraction is one well-known procedure for enhancing visualization of blood vessels to the exclusion of surrounding soft tissue and bony structures. In temporal subtraction, an x-ray image of the blood vessel of interest in the body is acquired just before an opaque x-ray contrast medium, such as an iodinated compound, that has been injected in the circulatory system arrives in the vessel. This is called a pre-contrast mask image and it contains the vessels and usually a background of soft tissue and bony structures. The pre-contrast image is usually digitized and the digital data representations of the picture elements (pixels) in the image are placed in a digital frame memory. When the contrast medium reaches the vessel of interest, an x-ray image is made and converted to digital data. The mask or pre-contrast image data are then subtracted from the post-contrast image data to cancel or subtract out all soft tissue and bony structure and anything that is common to both images to thereby enhance visualization of the blood vessel that contains the contrast medium. Usually the x-ray tube current and applied kilovoltages are the same for the pre-contrast and post-contrast images. The method is called temporal subtraction because of the substantial time lapse between the pre-contrast and post-contrast images. As is known, the pre-contrast mask images and post-contrast images will always have some noise content that is introduced by the x-ray system and the electronic components that are used to generate and process the signals that represent the images.
Temporal subtraction provides high signal-to-noise ratio (SNR) and is a preferred procedure in cases where there is little if any movement of soft tissue during the interval between acquisition of the pre-contrast and post-contrast images. However, when there is tissue motion there must necessarily be information that is not common to successive images. This results in a motion artifact which obscures the contrast of the contrast medium-filled vessel. Tissue movement is likely to exist in abdominal vessel studies wherein peristalsis of the digestive organs moves the vessels. Renal artery studies are often adversely affected. Movement is also exhibitied in carotid artery studies where the swallowing reflex causes an artifact which can obscure visualization of the vessels of interest.
Another image subtraction procedure is characterized as energy subtraction. Energy subtraction is based on the fact that x-ray attenuation by a body or any material is an x-ray energy dependent phenomenon and that the energy dependence is different for materials having different atomic number averages. In energy subtraction, an x-ray image of a region of interest in the body is obtained with a nominally low kilovoltage (kV) applied to the x-ray tube so the x-ray beam projected through the body has an average spectral distribution within a band having a low average energy. After a low average energy image is obtained and digitized, at least one more image is obtained with a comparatively higher kV applied to the x-ray tube and a resulting higher average energy spectral band. For angiographic studies, the two images are obtained when there is an x-ray contrast medium such as an iodinated compound present in the vessels. In any case, the high average energy image pixel data are subtracted from the low average energy pixel data and a difference image remains. Prior to subtraction, the data are usually variously weighted or scaled to bring about cancellation of soft tissue. The data could be scaled to eliminate bone from the difference image instead of tissue. However, it is not possible to remove or cancel bony structures without also removing most of the contrast medium which is really what one is trying to visualize in angiographic studies since it defines the interior shape of the vessel.
There are also brightness nonuniformities in the subtracted or difference images due to several effects when the image data are acquired using an image intensifier that is coupled to a television camera. Veiling glare, which is like haze, results from light diffusion or scattering often present in the input or output phosphors of the image intensifier. The fact that rays of a broad x-ray beam are scattered by body tissue in an energy dependent manner between ray paths also causes loss of contrast in the difference image. Differential detection of x-rays at various energies in the input phosphor of the image intensifier leads to additional brightness nonuniformities. None of these phenomena can be completely nullified by energy subtraction alone.
Hybrid subtraction has been proposed for cancelling or subtracting out stationary bone and soft tissue and for elimination of artifacts due to soft tissue movement while still providing an image of the contrast mediumfilled vessel. In hybrid subtraction, x-ray images are obtained using two x-ray spectra having different average energies and are combined in a manner to suppress signals due to soft tissue in a heterogeneous object such as the body. Basically, in one known hybrid subtraction procedure, a mask image is obtained first by projecting a low average energy x-ray beam (hereafter called low energy beam or low energy spectral band), through the body followed by a higher average energy x-ray beam (hereafter called high energy beam or high energy spectral band) when the injected x-ray contrast medium has not yet arrived in the blood vessel. The images, exhibiting primarily bone and soft tissue, acquired at two x-ray energies are scaled and weighted using appropriate constants and then subtracted to produce a mask image in which signals due to soft tissue variations are suppressed or cancelled and bony structures remain. The data for a pair of high and low energy x-ray images are next obtained after the injected contrast medium reaches the vessel in the region of interest. The data for this pair of images are acted upon, respectively, by the same constant weighting factors that are used with the first pair of pre-contrast medium images to cancel soft tissue and let bone and the contrast medium remain. One image acquired in this post-contrast medium exposure pair is subtracted from the other such that the resulting post-contrast difference image data contains data representative of the bone structures plus vessels containing contrast medium. The final step in hybrid subtraction is to subtract the dual energy post-contrast difference image from the dual energy pre-contrast difference image to effect the equivalent of temporal subtraction and thereby suppress or cancel the bone structures and isolate the contrast medium containing vessels. A major advantage of hybrid subtraction over temporal subtraction alone is the reduced sensitivity to soft tissue motion artifacts because the soft tissue is suppressed or cancelled in the pre-contrast and post-contrast dual energy images. Hybrid subtraction is superior for eliminating soft tissue structures that may have moved during the time between the mask image and post-contrast image or images. However, if there is no tissue movement, ordinary temporal subtraction is preferred because of its better signal-to-noise ratio compared to hybrid subtraction.
Another hybrid subtraction method has been reduced to the point of practical application by one of the inventors named herein. In this method, a sequence of pairs of low and high x-ray energy exposures are made over an interval comprised of a pre-contrast period when the contrast medium has not yet arrived in the vessel of interest, and a post-contrast period during which a substantial concentration of x-ray contrast medium has arrived and is flowing through the vessel, and an after-post-contrast period when substantially all of the contrast medium has departed from the vessel of interest. Usually, two to five but possibly as many as fifteen high and low x-ray energy exposure pairs are obtained per second. The x-ray images are converted to optical images with an image intensifier. A television camera converts the optical images to analog video signals. The analog video signals for each image frame are converted to digitally represented pixels. In the preferred one of several different available modes, the first low energy image frame data are used as mask image data. It is stored and all subsequent low energy image frame data in the exposure pairs are subtracted from the low energy mask image data to produce a series of low energy temporal difference images data which are stored on magnetic disk. The first high energy exposure in the sequence is also treated as a mask and all subsequent high energy exposure image frames are subtracted from the high energy mask alternatingly with the low energy subtractions and the resulting high energy difference images are stored alternatingly with the low energy images on magnetic disk. In the preferred procedure among those proposed by said inventor, the series of low energy temporal difference images and the series of high energy temporal difference images are accessed from the disk memory and are subjected to matched filtering wherein the data are multiplied by matched filter coefficients to emphasize the contrast medium signal of interest and filter out noise and other artifacts. The low and high matched filtered temporal difference images are then summed independently. The summation of the low energy filtered temporal difference image data are multiplied by a weighting constant, k.sub.L, and the summed high energy filtered temporal difference image data are multiplied by a weighting constant, k.sub.H. The constants are selected to bring about cancellation of anything that did not remain constant throughout the sequence of images and let the digital pixel data representative of the contrast medium filled vessel remain. After these multiplications are performed, the summed low and high energy image data are subtracted in what is called an energy subtraction step which results in a single frame of data wherein only pixel data representative of the shape of the contrast medium in the vessel remains. The advantage of the matched filtering approach is a substantial reduction in noise of the hybrid image. The invention described herein, characterized as integrated remasking, can also significantly reduce the noise in either temporal or hybrid subtraction. It additionally has the advantage of requiring somewhat less complex circuitry than matched filtering.